Isolation Transformer for 3-Port 3-Phase Dual-Active Bridge Converters in Medium Voltage Level

Abstract

In this paper, an isolation transformer with integrated filter inductances for three-phase three-port dual-active bridge (DAB) converters in the wye-wye-delta (Yyd) configuration is introduced and designed. A large number of ports and phases in the application necessarily requires a proportionally increased number of components, accessories, and connections. These additional parts induce significant losses and electromagnetic interference during high-frequency operations. Hence, fully manipulating the parasitic components, especially the leakage inductances of the transformer as the circuit element in the interconnected multi-port configuration, is a key to reduce the system's overall size and to improve its reliability. The proposed geometry and design method enables the full integration of a large number of otherwise bulky inductors to be included in the isolation transformers so that the latter function not only a step-up/down transformers but also as filter networks required for three-port DAB operations. The transformer is suitable for high-power and high step-up/down ratio DC-DC converters, which prefers a parallel combination of converters that share current, on the low-voltage side. The operating principles and steady-state analysis are presented with respect to power flow, and a three-winding shell-type isolation/filter transformer has been designed for a three-port three-phase Yyd DAB converter for solid state transformer applications. The finite element method simulations are used to validate the feasibility of the proposed approach. A prototype was fabricated and tested in an experimental setting.